1. Field of the Invention
The present invention relates to a discharge device suitable for use in an ignition system with a series gap for automotive engines, and an ignition system with a series gap using the discharge device.
2. Description of the Prior Art
There is known in the prior art what is called a discharge tube used as a discharge device wherein a pair of electrodes are disposed on opposite sides in a sealed tube and an inactive gas is sealed in the sealed tube.
FIG. 22 shows a discharge tube 1 used in for example an arrester for various kinds of electrical machinery and apparatus. In this discharge tube 1, the open sections at both ends of an insulation tube 2 as a sealed tube formed of ceramics or other are closed with a pair of electrode plates 4, 4 having discharge electrode sections 3, 3 which are disposed opposingly. In the insulation tube 2 is sealed a specific inactive gas. When overvoltage resulting from lighting or other has entered the device, there occurs an electric discharge between the discharge electrode sections 3, 3, momentarily energizing the discharge tube 1 to discharge the overcurrent to the ground side.
Since the discharge tube 1 is placed and connected on a specific electric circuit of an arrester for example, there is arranged an electric element having a specific potential or a metal at a ground potential. If, in this manner, the discharge tube 1 is installed close to an electric element, an electric field formed around the electric element or other will affect the discharge tube 1, resulting in changed discharge voltage characteristics of the discharge tube 1.
There is also known as the prior art (Japanese Patent Publication No. 51-32180) an ignition system C for automotive engines which is provided with a so-called series gap S disposed in series with the spark plug 5 as shown in FIG. 23 in order to prevent the fouling of the spark plug 5 coated with carbon soot, thereby maintaining constant ignition timing. The formation of the series gap S of the discharge tube 1 having the aforementioned constitution is considered.
The ignition system C with series gap as shown in FIG. 24, has the spark-tension cable 6 connected to the high-voltage distribution side. In the plug gap 7 are opposingly disposed a connection terminal 9 designed to fit on a terminal 8 of the spark plug 5 and a high-voltage distribution terminal 10 connected to the end of the high-tension cable 6. And between the terminals 9 and 10 the discharge tube 1 having the discharge electrodes 3, 3 are mounted. The ignition system C of the above constitution is mounted in a recess section 12 formed in an engine cylinder head or a cylinder head cover 11, and attached on the spark plug 5 which is screwed into the cylinder head side. In the drawing, numeral 13 denotes a metal pipe which has the purpose of guiding the ignition system C and also protecting the ignition system C from fouling by engine oil.
However, since the discharge tube 1 is built in the ignition system C and inserted in the cylinder head or the cylinder head cover 11 as described above, the metal pipe 13 which guides the ignition system C is located close thereto. In addition, since the metal pipe 13 is at the same potential as the ground potential of the cylinder head or the cylinder head cover 11, the presence of the metal pipe 13 at this ground potential changes the field strength around the discharge tube 1, resulting in changed discharge voltage characteristics of the discharge tube 1. Such a change in the discharge voltage characteristics is likely to shift the whole ignition timing of the spark plug 5, and accordingly there is the problem that it is impossible to obtain specific automotive engine performance in which the accurate control of the spark plug ignition timing is required to obtain a high engine performance.